Boring interjection:
Yes, there posibly is slate under the thatch on the offshot - the bedroom opened up to the roof apex will probably have a slate base, and if thatched with water reeds, then it may all be slate underneath due to building regs. Why do I know this? I spend hours perusing the property market.
Anyway, for those who are scared the murals will be lost - this cottage may well go to someone seeking to have it as a holiday home and let it out, in which case, murals like that would be a huge attraction for holidaymakers. Though I would be
very scared of spraying my cooking up the walls and ruining them.
I wonder if this is close to where Alan Lee lives? He says he gets his inspiration from the local landscape - check out his illustration of Tom & Goldberry's house (in the illustrated LotR), it's very similar to this one, and he said he drew it as a traditional Devon cottage.